1. Field of the Invention
The invention relates to a solar cell lead, in particular, to a solar cell lead excellent in bondability to a solar cell, a method of manufacturing the same and a solar cell using the same.
2. Description of the Related Art
In a solar cell, polycrystalline and single crystal Si cells are used as a semiconductor substrate.
As shown in FIGS. 6(a) to (c), a solar cell 100 is manufactured by bonding solar cell leads 103a and 103b to a predetermined region of a semiconductor substrate 102, i.e., to a front surface electrode 104 provided on a front surface of the semiconductor substrate 102 and to a back surface electrode 105 provided on a back surface thereof, using a solder or an adhesive. Electricity generated in the semiconductor substrate 102 is transmitted to the outside through a solar cell lead 103.
As shown in FIG. 8, a conventional solar cell lead 103 is provided with a strip plate conductive material 112 and hot-dip solder plating layers 113 formed on upper and lower surfaces of the strip plate conductive material 112. The strip plate conductive material 112 is formed by, e.g., roll processing a circular cross-section conductor into a ribbon shape, which is called a flat conductor or a flat wire.
The hot-dip solder plating layer 113 is formed by supplying a molten solder on the upper and lower surfaces of the strip plate conductive material 112 using a hot dipping method.
The hot dipping method is a method in which an upper surface 112a and a lower surface 112b of the strip plate conductive material 112 are cleaned by acid pickling, etc., and a solder is laminated on the upper surface 112a and the lower surface 112b of the strip plate conductive material 112 by passing the strip plate conductive material 112 through a molten solder bath. As shown in FIG. 8, the hot-dip solder plating layer 113 is formed in a shape bulging from a side portion in a width direction to a center portion, so-called a mountain-like shape, by an effect of surface tension at the time of solidification of the molten solder adhered on the upper surface 112a and the lower surface 112b of the strip plate conductive material 112.
In the conventional solar cell lead 103 shown in FIG. 8, the hot-dip solder plating layers 113 bulged in a mountain-like shape are formed on the upper and lower surfaces 112a and 112b of the strip plate conductive material 112. In the solar cell lead 103, since the hot-dip solder plating layer 113 is bulged in a mountain-like shape, it is difficult to obtain a stable laminated state at the time of winding around a bobbin, and deformation of the winding is likely to occur. In addition, a lead wire may be tangled due to the deformation of the winding, and may not be pulled out.
The solar cell lead 103 is cut to a predetermined length, is sucked up by air suction and moved onto a front surface electrode 104 of the semiconductor substrate 102 of FIG. 6, and is bonded to the front surface electrode 104 of the semiconductor substrate 102 by using a solder or an adhesive. An electrode band (not shown) electrically conducting with the front surface electrode 104 is preliminarily formed on the front surface electrode 104. The hot-dip solder plating layer 113 of a solar cell lead 103a is brought in contact with the front surface electrode 104, then, bonding is carried out by soldering or preliminary applying an adhesive. The bonding of a solar cell lead 103b to a back surface electrode 105 of the semiconductor substrate 102 is carried out in the same way.
At this time, since the hot-dip solder plating layer 113 of the solar cell lead 103a of FIG. 8 is bulged and the thickness is uneven, a contact area thereof with the an air suction jig is small and a suction force is not sufficient, hence, there is a problem of a fall during the moving operation. In addition, a contact area of the front surface electrode 104 with the hot-dip solder plating layer 113 becomes small. When the contact area of the front surface electrode 104 with the hot-dip solder plating layer 113 is small, heat conduction from the semiconductor substrate 102 to the hot-dip solder plating layer 113 is not sufficient, which results in generation of a soldering defect.
In addition, the small contact area of the front surface electrode 104 with the hot-dip solder plating layer 113 causes a misalignment between the solar cell lead 103a soldered to the front surface electrode 104 and the solar cell lead 103b soldered to the back surface electrode 105 when jointing the solar cell leads 103a and 103b to both the front and back surfaces of the semiconductor substrate 102, and a cell crack (which means that the semiconductor substrate 102 is cracked) occurs due to the misalignment. Since the semiconductor substrate 102 is expensive, a cell crack is unfavorable.
A method has been proposed in which a solder-plated wire is rolled, cut and etched for forming a concavo-convex portion on solder plating in order to increase a contact area of the front surface electrode 104 with the hot-dip solder plating layer 113 which allows rapid heating, and a decrease in module output is suppressed by increasing a contact area with a surface electrode, thereby increasing reliability (WO 2006/128204 and JP-A 2008-147567),
As shown in FIG. 7, a solar cell lead 93 of WO 2006/128204 and JP-A 2008-147567 has a concavo-convex portion formed by rolling, cutting or etching a hot-dip solder plating layer which is formed on upper and lower surfaces of a strip plate conductive material. When such a solar cell lead is soldered to a front or back surface electrode of a semiconductor substrate via a hot-dip solder plating layer, the solar cell lead is tightly bonded to the semiconductor substrate and is less likely to separate therefrom, thus, excellent in durability.
As described above, according to the solar cell lead of WO 2006/128204 and JP-A 2008-147567, since the solder plating on the upper and lower surfaces is concavo-convex patterned and the plating layer is flat from lateral to middle portion in a width direction, misalignment with respect to the electrode is less likely to occur and a cell crack due to the misalignment is less likely occur. However, since the cell is likely to warp due to temperature change during the bonding to the electrode by a solder or a resin and the lead wire is thus likely to separate, there is still a problem that the module output decreases.
Thinning of semiconductor substrate has been examined since the most part of the cost of the solar cell is spent on a semiconductor substrate, however, a thinned semiconductor substrate is likely to warp and crack during bonding to an electrode. A cell crack due to warpage or separation of flat conductor is more likely to occur when a thickness of the semiconductor substrate is, e.g., 200 μm or less. If the decrease in module input is generated due to the cell cracks or the separation of flat conductor caused by a solar cell lead, the thinning of semiconductor substrate is not expected.